Why do we prefer files supplied as CMYK?
They are totally different colour systems.
RGB
RGB is a colour system used on screens, it uses light to create your colours, this results in bright colours. It stands for Red; Green; Black, your colour is created based on the values you assign each.
255 is the maximum value you can use, so 255 Red would be a bright vibrant Red; 255 Green is a vibrant Lime Green, etc. To create White you use the maximum amount or R, G and B, and for Black you don’t use any colour.
CMYK
CMYK is how what is used on printing presses as it is the colour of the inks that are used. It works in the opposite way, in that you add colours to create new colours but it can result in duller colours. It stands for Cyan, Magenta, Yellow and Black. To create a vibrant Red you would use 100% Magenta and 100% Yellow. To create White you don’t use any colour, and Black can be either 100% Black or 100% of C, M, Y and B would produce a very dense Black.
Examples
Here are some examples of how colours can look totally different, of course the difficulty is that you are viewing these colours on a screen, so they will not truly represent the final colour. It is very confusing!
The first green is a RGB Lime Green, next to it is the colour you get when you convert it to CMYK.
Unfortunately you will never be able to produce a Lime Green using CMYK, you would have a Pantone colour instead, which is a specially prepared ink colour.
Pink is another one where the colour becomes duller in RGB, as demonstrated below.
Add to this the fact that everyone’s screens are set up differently, whether it is a Desktop machine, Tablet or mobile phone, the same colour will look slightly different. For this reason we will never be able to match what you see on your own device, but at least if your file is CMYK it will give you a better representation.
The only difference between these two images is one is RGB, the other CMYK!
If you are all unsure of your colours, always ask for a printed proof.